A storage system typically comprises one or more storage devices into which information may be entered, and from which information may be obtained, as desired. The storage system includes a storage operating system that functionally organizes the system by, inter alia, invoking storage operations in support of a storage service implemented by the system. The storage system may be implemented in accordance with a variety of storage architectures including, but not limited to, a network-attached storage environment, a storage area network and a disk assembly directly attached to a client or host computer. The storage devices are typically disk drives organized as a disk array, wherein the term “disk” commonly describes a self-contained rotating magnetic media storage device. The term disk in this context is synonymous with hard disk drive (HDD) or direct access storage device (DASD).
Storage of information on the disk array is preferably implemented as one or more storage “volumes” of physical disks, defining an overall logical arrangement of disk space. The disks within a volume are typically organized as one or more groups, wherein each group may be operated as a Redundant Array of Independent (or Inexpensive) Disks (RAID). Most RAID implementations enhance the reliability/integrity of data storage through the redundant writing of data “stripes” across a given number of physical disks in the RAID group, and the appropriate storing of redundant information (parity) with respect to the striped data. The physical disks of each RAID group may include disks configured to store striped data (i.e., data disks) and disks configured to store parity for the data (i.e., parity disks). The parity may thereafter be retrieved to enable recovery of data lost when a disk fails. The term “RAID” and its various implementations are well-known and disclosed in A Case for Redundant Arrays of Inexpensive Disks (RAID), by D. A. Patterson, G. A. Gibson and R. H. Katz, Proceedings of the International Conference on Management of Data (SIGMOD), June 1988.
The storage operating system of the storage system may implement a high-level module, such as a file system, to logically organize the information stored on the disks as a hierarchical structure of named data containers, such as directories, files and blocks. For example, each “on-disk” file may be implemented as set of data structures, i.e., disk blocks, configured to store information, such as the actual data for the file. These data blocks are organized within a volume block number (vbn) space that is maintained by the file system. The file system organizes the data blocks within the vbn space as a “logical volume”; each logical volume may be, although is not necessarily, associated with its own file system. The file system typically consists of a contiguous range of vbns from zero to n, for a file system of size n−1 blocks.
A known type of file system is a write-anywhere file system that does not overwrite data on disks. If a data block is retrieved (read) from disk into a memory of the storage system and “dirtied” (i.e., updated or modified) with new data, the data block is thereafter stored (written) to a new location on disk to optimize write performance. A write-anywhere file system may initially assume an optimal layout such that the data is substantially contiguously arranged on disks. The optimal disk layout results in efficient access operations, particularly for sequential read operations, directed to the disks. An example of a write-anywhere file system that is configured to operate on a storage system is the Write Anywhere File Layout (WAFL™) file system available from Network Appliance, Inc., Sunnyvale, Calif.
The storage operating system may further implement a storage module, such as a RAID system, that manages the storage and retrieval of the information to and from the disks in accordance with input/output (I/O) operations. The RAID system is also responsible for parity operations performed on the RAID groups in the storage system. The RAID system typically organizes the RAID groups into one large “physical” disk (i.e., a physical volume), such that the disk blocks are concatenated across all disks of all RAID groups. The logical volume maintained by the file system is then “disposed over” (spread over) the physical volume maintained by the RAID system.
The storage system may be configured to operate according to a client/server model of information delivery to thereby allow many clients to access the directories, files and blocks stored on the system. In this model, the client may comprise an application, such as a database application, executing on a computer that “connects” to the storage system over a computer network, such as a point-to-point link, shared local area network, wide area network or virtual private network implemented over a public network, such as the Internet. Each client may request the services of the file system by issuing file system protocol messages (in the form of packets) to the storage system over the network. By supporting a plurality of file system protocols, such as the conventional Common Internet File System (CIFS) and the Network File System (NFS) protocols, the utility of the storage system is enhanced.
When accessing a block of a file in response to servicing a client request, the file system specifies a vbn that is translated by the RAID system into a physical block number (pbn) location on a particular disk (disk, pbn) within a RAID group of the physical volume. Each block in the vbn space and in the pbn space is typically fixed, e.g., 4 k bytes (kB), in size; accordingly, there is typically a one-to-one mapping between the information stored on the disks in the pbn space and the information organized by the file system in the vbn space. The (disk, pbn) location specified by the RAID system may be further translated by a disk driver system of the storage operating system into a plurality of sectors on the specified disk.
The requested block is then retrieved from disk and stored in a buffer cache of the memory as part of a buffer tree of the file. The buffer tree is an internal representation of blocks for a file stored in the buffer cache and maintained by the file system. Broadly stated, the buffer tree has an inode at the root (top-level) of the file. An inode is a data structure used to store information, such as metadata, about a file, whereas the data blocks are structures used to store the actual data for the file. The information contained in an inode may include, e.g., references to locations on disk of the data blocks for the file. The references to the locations of the file data are provided by block pointers, which may further reference indirect blocks that, in turn, reference the data blocks, depending upon the quantity of data in the file.
The RAID system maintains information about the geometry of the underlying physical disks (e.g., the number of blocks in each disk) in data structures, such as raid labels, stored on the disks. The RAID system provides the disk geometry information to the file system for use when creating and maintaining the vbn-to-disk, pbn mappings used to perform write allocation operations and to translate vbns to disk locations for read operations. Block allocation data structures, such as an active map, a snapmap, a space map and a summary map, are data structures that describe block usage within the file system, such as the write-anywhere file system. These mapping data structures are independent of the geometry and are used by a write allocator of the file system as existing infrastructure for the logical volume.
Data storage is a central part of many industries that operate in archival and compliance application environments, such as banks, government facilities/contractors and securities brokerages. In many of these environments, it is necessary to store selected data, e.g., e-mails, financial documents and transaction records, in an immutable and unalterable manner, possibly for long periods of time. Typically, data backup operations are performed to ensure the protection and restoration of such data in the event of failure. However, backup operations often result in the duplication of data on backup storage resources, such as disks, causing inefficient consumption of storage space on those resources. It is thus desirable to eliminate duplicate data on the storage resources and ensure the storage of only single instances of data to thereby achieve storage compression.
One known approach to providing data storage compression involves a content addressable storage system that utilizes a hash to determine whether or not data to be written previously exists on backup storage resources. The process of computing the hash in the content addressable storage system typically takes place at the file level (as opposed to the logical unit or lun level) and is relatively slow because of the substantial computation needed to determine whether or not the data has already been stored. Consequently, write operations are typically slow processes; this is the bane of content addressable storage systems in the industry.
Another known approach is the Carnegie Mellon University (CMU) Network Attached Secure Disk (NASD) system developed by the CMU Parallel Development Lab. The NASD involves “smart” disks attached to a network as a way of replacing direct attached disks. However, as in the case of a general purpose system, this approach suffers from performance that is not sufficient for primary storage and from the use of public networks that are not sufficiently reliable to provide predictable performance.